The present invention relates to a method for estimating scattered radiation, whose main application is intended for the correction of radiographs.
The utilisation of a beam of rays associated with a two-dimensional sensor, very frequently found in radiography, has the disadvantage of producing significant scattered radiation on the radiograph through the subject under examination. In other terms, each of the detectors located behind the subject receives not only a primary radiation, arriving directly from the source in a straight line and having crossed through a well-defined region of the subject, but also scattered radiation of indeterminate origin which disturbs the measurement and which it is therefore desirable to correct.
Several methods are already in use. Thus, the primary radiation can be measured alone if there is strict collimation of the detectors and the source in order to intercept the scattered radiation, but this method, in practice, needs a sweeping of the beam which is slow to carry out, and during which the movements of the patient must be taken into account if one is examining living beings.
The opposite idea has also been suggested, measuring only the scattered radiation. For this, one uses a discontinuous network of absorbers, such as lead balls, between the object and the detectors, to stop the primary radiation locally, such that the detectors located behind these absorbers only measure the scattered radiation. This method, called xe2x80x9cbeam stopxe2x80x9d thus provides two-dimensional tables or maps for the value of the scattered radiation, which are completed by interpolation between the detectors placed behind the absorbers. The diffused radiation thus estimated is subtracted from the total radiation measured separately. This method is precise but has the disadvantage of imposing two irradiations of the subject and thus doubling the dose of radiation it receives. A final example of a method for correcting scattered radiation by material means includes the use of anti-scatter grids, but their efficiency is only partial; it is insufficient for a conical beam, where the scattered radiation can be several times higher than the primary radiation.
Finally, there are a certain number of digital methods for estimating the scattered radiation, for example from convolutions or de-convolutions of measurements; one can also mention the French patent 2 759 800 as an analytic, different digital method. In general, they are complicated to use because they depend on the parameters chosen by the operator (convolution cores, for example) which only provide good results under favourable circumstances, such as small zones where the scattered radiation is low, or objects with relatively homogeneous content. No simple method exists making it possible, for example, to correct the scattered radiation through the thorax or other large anatomical zones, which are frequently examined but which are difficult for correcting the scattered radiation because of their volume and the heterogeneity due to the presence of a complex bone structure whose radiation attenuation capacity is very different from that of soft tissues.
Finally one should mention the U.S. Pat. No. 6,018,565 for the description of a mixed method, with xe2x80x9cbeam stopxe2x80x9d and convolution.
An essential aim of the present invention is to propose a method for estimation and correction for scattered radiation which can suit difficult radiography situations.
The method according to the invention is, in its most general form, a method for estimating scattered radiation coming from an initial radiation having crossed an object and undergone an attenuation allowing a total measurement radiation to pass, characterised by:
drawing up a table of measurements of scattered radiation, obtained by making the initial radiation pass through a simulacrum of the subject,
calculation of transposition coefficients between the simulacrum and the subject, according to the initial radiation, the total measurement radiation through the subject and a total measurement radiation through the simulacrum,
and a weighting of the measurement table with the transposition coefficients.
Advantageously, the simulacrum will be a block of constant thickness and of a homogeneous material, having an attenuation similar to a base material of the subject; in general the measurement table will be drawn up from a selection in a series of measuring tables for scattered radiation, obtained beforehand by passing the initial radiation successively through a respective series of simulacra of the subject, of different but constant thicknesses; the selection will be made by comparison of a total measurement radiation value through the subject and a total measurement radiation value through the simulacra.
The weighting coefficients are generally value ratios of a same functional calculated for the subject and for the simulacrum. The functional used can be equal to the product of the total measurement radiation by the logarithm of the ratio of total measurement radiation and the initial radiation.